1. Field of the Invention
The present invention relates to a precision driving apparatus of the type comprising a rack and a pinion meshed with each other. Such precision driving apparatus is used, for example, in surveying instruments to move the focusing lens thereof along the optical axis. The present invention is directed to improvements in the structure of the rack of such precision driving apparatus.
2. Description of the Prior Art
As is well known to those skilled in the art, adjustment of engagement between pinion and rack in a precision driving mechanism is very troublesome and requires considerable time and labour. Too much play in meshing results in chattering whereas too close meshing results in grating. Even after a proper adjustment has been once made, the engagement becomes loose with use for a long time and readjustment is required.
A known solution to the above mentioned problem is to provide a notch extending along most of the length of the rack on a surface of the rack opposite to the toothed surface thereof so as to give the rack resiliency to some extent. According to the solution, a better engagement is assured between rack and pinion at the middle portion of the rack where it is allowed to flex. However, at the both end portions of the rack there is assured no satisfactory engagement between the two members because of lack of flexure of the rack at the end portions. There, the engagement becomes worse and generally the teeth grate on each other. Although the rack can flex at the middle portion, it is generally difficult to give the rack adequate flexture by this solution. Even when this solution was employed, a fine adjustment was still required at the time of assembly.